Annalee Newitz

It turns out that trees share a fundamental property with the universe itself. The the older they get, the more quickly they expand. A new study reveals that older trees grow more quickly than their younger counterparts. They are better carbon sinks, too. But why?

An international group of ecological and plant biologists conducted a massive study of 403 ancient trees around the world. They wanted to figure out how older trees contribute to the carbon cycle, where trees take in atmospheric carbon and release oxygen. What they discovered went against all the rules of aging that we've learned in the animal world.

It turns out that older trees are the most powerful elements in the carbon-fixing machine that is the forest. Not only do older trees grow bigger more quickly than younger ones, but they also fix carbon at a much more prodigious rate.

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Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree.

So one big, old tree can remove the same amount of carbon from the atmosphere in one year that a mid-sized tree has removed in its entire lifetime. Of course if that mid-sized tree makes it to old age, it too could start packing down the carbon as well.

But there are still questions. As trees age, their leaves — which take in carbon — aren't as productive as they once were. Plus, the tree's community of similar trees in the forest, called a stand, becomes less productive overall too. So how do individual, older trees become so powerful? The scientists suggest that the rapid growth of the older tree means that it has more leaves, or more "total leaf area," than younger trees. So each leaf is less productive, but there are more leaves overall contributing to the carbon sink. Meanwhile, as the tree stand ages, it becomes less dense as the trees in it die out or are removed over time. So the remaining aged trees have more access to sunlight and other nutrients as their stand neighborhood loses density.

What's tragic about this is that logging in the Americas targeted large, old trees first. So we've lost many of the carbon-sink powerhouses that we had 100 years ago. In the future, foresters may want to target younger trees for cutting, preserving the older ones because they help cleanse the environment more effectively.